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Journal of Physics B Atomic Molecular and Optical Physics

Measurements on impulsive force-induced dynamics of a cold85Rb atom cloud in a magneto-optical trap

Vibhuti Bhushan Tiwari, Sunita Singh, H. S. Rawat, Manoranjan P. Singh

Published Oct 1, 2008• 6 citations
Publisher

Abstract

We report on the study of impulsive force-induced dynamics of a cold 85Rb atom cloud in a magneto-optical trap. The impulse, imparted by a laser beam of variable pulsewidth, forces the atom cloud away from the centre of the trap. Depending on the initial drift velocity of the atom cloud, we observe either oscillation or escape of the cloud. We investigate these processes as a function of the trap parameters, especially around the onset of the multiple scattering regime.

DOI
10.1088/0953-4075/41/20/205301

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We report on the study of impulsive force-induced dynamics of a cold 85Rb atom cloud in a magneto-optical trap. The impulse, imparted by a laser beam of variable pulsewidth, forces the atom cloud away from the centre of the trap. Depending on the initial drift velocity of the atom cloud, we observe either oscillation or escape of the cloud. We investigate these processes as a function of the trap parameters, especially around the onset of the multiple scattering regime.
We have observed self-sustained radial oscillations in a large magneto-optical trap, containing up to 10(10) Rb85 atoms. This instability is due to the competition between the confining force of the magneto-optical trap and the repulsive interaction associated with multiple scattering of light inside the cold atomic cloud. A simple analytical model allows us to formulate a criterion for the instability threshold, in fair agreement with our observations. This criterion shows that large numbers of trapped atoms N>10(9) are required to observe this unstable behavior.
This paper presents an experimental demonstration of light-induced evaporative cooling in a magneto-optical trap. An additional laser is used to interact with atoms at the edge of the atomic cloud in the trap. These atoms get an additional force and evaporated away from the trap by both the magnetic field and laser fields. The remaining atoms have lower kinetic energy and thus are cooled. It reports the measurements on the temperature and atomic number after the evaporative cooling with different parameters including the distance between the laser and the centre of the atomic cloud, the detuning, the intensity. The results show that the light-induced evaporative cooling is a way to generate an ultra-cold atom source.
Large clouds of cold atoms prepared in a magneto-optical trap are known to present spatiotemporal instabilities when the frequency of the trapping lasers is brought close to the atomic resonance. This system bears similarities with trapped plasmas where the role of the Coulomb interaction is played by the exchange of scattered photons, and astrophysical objects, such as stars whose size is dependent on radiative forces. We present in this paper a study of the phase space of such instabilities and reveal different dynamical regimes. Three-dimensional simulations of the highly nonlinear atomic dynamics permit a detailed analysis of the experimental observations.
Nonlinear oscillations and synchronization phenomena are ubiquitous in nature. We study the synchronization of self-oscillating magneto-optically trapped cold atoms to a weak external driving. The oscillations arise from a dynamical instability due the competition between the screened magneto-optical trapping force and the interatomic repulsion due to multiple scattering of light. A weak modulation of the trapping force allows the oscillations of the cloud to synchronize to the driving. The synchronization frequency range increases with the forcing amplitude. The corresponding Arnold tongue is experimentally measured and compared to theoretical predictions. Phase locking between the oscillator and drive is also observed.
We experimentally investigate one-dimensional (1D) expansion of a cold cloud of cesium atoms in orthogonal 2D configuration of near resonant laser beams by temporally modulating a pair of counterpropagating trapping beams of a magneto-optical trap (MOT). The cloud is observed to undergo ballistic expansion followed by superballistic explosive growth due to the fluctuations of the 2D radiation force. A model based on the theory of Brownian motion is developed and a comparison of experiments with theory is shown to provide a direct measure of the velocity diffusion coefficient. We also observe sudden contraction of the cloud immediately after switching off the pair of trapping beams, which provides direct evidence for the existence of the radiation trapping force in a MOT.
We present an experimental set-up for the production of a beam source of cold 87Rb atoms, which is formed from a trapped cold atomic cloud in a three-dimensional (3D) magneto-optical trap (MOT). Via a radiation pressure difference generated by a specially designed leak tunnel along one trapping laser beam, the atoms are pushed out continuously with low velocity and high flux. The beam was measured to have a mean velocity of 12m/s and a longitudinal velocity spread of about 3m/s under nominal experimental conditions, based on the fluorescence and the time of flight (TOF) technologies. And its flux can be tuned by adjusting the MOT parameters from 108 to 109 atoms/s. Especially the beam performance dependence on trapping laser detuning and power is experimentally studied.